Question

How many among the given species have the highest bond order? CN\(^-\), CO, NO\(^-\), O\(_2\), N\(_2\)

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Remember, bond order is calculated by the number of bonding electrons minus anti-bonding electrons, divided by 2. The species with the highest bond order typically have more bonding electrons and fewer anti-bonding electrons.

Answer 1

The Correct Option is C

To calculate bond order, we use the formula: \[ \text{Bond Order} = \frac{\text{(Number of bonding electrons)} - \text{(Number of anti-bonding electrons)}}{2} \] We calculate the bond order for each species based on their molecular orbital (MO) diagram.
1. CN\(^-\):
The molecular orbital configuration for CN\(^-\) (an extra electron) is: \[ 1\sigma_g^2, 1\sigma_u^2, 2\sigma_g^2, 2\sigma_u^2, 1\pi_u^4, 1\pi_g^2 \] The number of bonding electrons = 8, anti-bonding electrons = 4, so: \[ \text{Bond Order} = \frac{8 - 4}{2} = 2 \] 2. CO: The molecular orbital configuration for CO is: \[ 1\sigma_g^2, 1\sigma_u^2, 2\sigma_g^2, 2\sigma_u^2, 1\pi_u^4, 1\pi_g^2 \] The number of bonding electrons = 10, anti-bonding electrons = 4, so: \[ \text{Bond Order} = \frac{10 - 4}{2} = 3 \]
3. NO\(^-\):
The molecular orbital configuration for NO\(^-\) (an extra electron) is: \[ 1\sigma_g^2, 1\sigma_u^2, 2\sigma_g^2, 2\sigma_u^2, 1\pi_u^4, 1\pi_g^3 \] The number of bonding electrons = 9, anti-bonding electrons = 5, so: \[ \text{Bond Order} = \frac{9 - 5}{2} = 2 \]
4. O\(_2\): The molecular orbital configuration for O\(_2\) is:
\[ 1\sigma_g^2, 1\sigma_u^2, 2\sigma_g^2, 2\sigma_u^2, 1\pi_u^4, 1\pi_g^2 \] The number of bonding electrons = 10, anti-bonding electrons = 6, so: \[ \text{Bond Order} = \frac{10 - 6}{2} = 2 \]
5. N\(_2\):
The molecular orbital configuration for N\(_2\) is: \[ 1\sigma_g^2, 1\sigma_u^2, 2\sigma_g^2, 2\sigma_u^2, 1\pi_u^4, 1\pi_g^2 \] The number of bonding electrons = 10, anti-bonding electrons = 4, so: \[ \text{Bond Order} = \frac{10 - 4}{2} = 3 \] Thus, CO and N\(_2\) have the highest bond order of 3.
Therefore, the correct answer is (3) 3.